1. Field of the Invention
This invention relates to a liquid level indicator.
2. Description of the Prior Art
It is known from, for example, U.S. Pat. No. 3,272,174 to provide an indication of the liquid level in a tank by directing light onto the liquid surface from a light source positioned above the liquid level. The incident light is partly reflected from the surface of the liquid and a small proportion of the reflected light is intercepted by the ends of optical fibres located at different heights within the tank. Only those fibres which are positioned above the liquid surface are then illuminated to provide an indication of the liquid level.
In practice, however, the known device described in the preceding paragraph suffers from a number of disadvantages. In particular, the device produces a relatively low light output since the design of the device is such that much of the light from the light source is lost rather than being reflected into the optical fibres used to produce the display. Similarly, there is a tendency for light to be scattered by the internal walls of the tank thereby resulting in a low contrast ratio between the illuminated and non-illuminated display. Moreover, the arrangement of the device is such that the display in fact indicates how empty the tank is, since it will be appreciated from the preceding discussion that more output fibres are illuminated as the tank empties. Further, the device tends to produce a flickering output when used to monitor the liquid level in a moving tank such as, for example, a road vehicle fuel tank, as a result of agitation of the liquid in the tank.
An alternative method of liquid level detection, as disclosed in, for example, U.S. Pat. No. 3,120,125, involves transmitting light from a first optical fibre to a second optical fibre by total internal reflection at the dielectric interface between a transparent medium and the vapor above the liquid in a tank. When the liquid level rises above said interface, the higher refractive index of the liquid as compared with that of the vapor prevents the total internal reflection so that transmission of the light between the first and second optical fibres ceases. A plurality of separate pairs of said first and second optical fibres are mounted at varying heights respectively within the tank. However, although this arrangement produces an improved light output with less scatter than the device disclosed in U.S. Pat. No. 3,272,174, it still suffers from the disadvantages that it indicates how empty, rather than how full the tank is, and that it produces a flickering output when used to monitor the liquid level in a moving tank.